Koinly can sync your Coinbase transactions using oAuth. Go to the Wallets page on Koinly and click on Add Wallets, then select Coinbase. This will take you to Coinbase's website where you can approve Koinlys access to your transactions. Note that Koinly only asks for read-access to the transactions in your account.
If you also trade on Coinbase Pro, you must add the Coinbase Pro wallet separately. Your Coinbase wallet does not give Koinly access to your transactions on Coinbase Pro (GDAX) and vice versa.
How can I add multiple accounts?
You can add as many Coinbase accounts as you like, just follow the instructions above for each account.
Why do I have to add Coinbase Pro?
Your Coinbase and Coinbase Pro accounts are linked together internally but Koinly does not automatically get access to your Coinbase Pro transactions when you add a Coinbase wallet so it's important to add it separately.
Why am I seeing transactions from Coinbase Pro to Coinbase even though I never sent them?
Whenever you transfer crypto from Coinbase Pro, it is internally sent to your Coinbase account first before being sent out to the destination address. These transactions are not taxable events so they wont affect your taxes.